1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern. For example, the patterning means can be, but is not limited to, a reflective or transmissive contrast device, such as a spatial light modulator, a digital mirror device, a grating light valve, a liquid crystal display, or the like.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing a pattern onto the target portion in each exposure period. Other known lithographic apparatus include scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
A metrology target generally refers to a type of target that may form part of the pattern written to the substrate, but which does not actually contribute directly to the functional or structural form of the device being manufactured. Usually, the function of a metrology target is to facilitate aspects of the manufacturing process itself, such as alignment of a substrate to the projection system, verification of overlay and/or imaging properties, etc. Metrology targets may therefore include alignment marks and targets used in “offline” metrology equipment associate with or within the lithography apparatus. Offline generally refers to metrology equipment designed to process a substrate separately from, and at a different time to, the main lithography processes used to pattern the substrate, while inline metrology refers to processes carried out at the same time and/or position. For the purposes of this description, protective structures for the above alignment marks and targets are themselves to be understood as types of metrology target.
In one example using mask-based systems, the metrology targets normally have to be defined before the mask is actually made. If it turns out that in manufacturing conditions the metrology target design is non-optimal, e.g., for overlay performance, a new mask or set of masks has to be produced before an improved metrology target design can be implemented. This hampers the speed at which the potential of new metrology target designs can be evaluated and leads to increased costs for the customer.
Therefore, what is needed is a system and method that can optimize performance of metrology targets in lithographic devices. Additionally or alternatively, what is needed is a system and a method that can add flexibility in the choice of metrology target even after the product design has been finalized.